Fuel cells are capable of continuously generating electric power through continuous supply of a fuel (e.g., hydrogen) and an oxidizing agent (e.g., oxygen). Unlike primary batteries (such as dry batteries) and secondary batteries (such as lead storage batteries), the fuel cells generate electric power at a high power generating efficiency without being significantly affected by the scale of an electric-power-using system and do not generate much noise and vibrations. The fuel cells are therefore expected to be used as energy sources covering a wide variety of uses and scales. Specifically, the fuel cells have been developed as polymer electrolyte fuel cells (PEFCs), alkaline fuel cells (AFCs), phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs), solid oxide fuel cells (SOFCs), and biofuel cells. Among them, polymer electrolyte fuel cells have been developed for use in fuel cell powered vehicles, domestic use fuel cells (domestic use co-generation systems), and mobile devices such as cellular phones and personal computers.
Such a polymer electrolyte fuel cell (hereinafter simply referred to as a “fuel cell”) includes a stack of plural single cells, in which each single cell includes an anode, a cathode, and a polymer electrolyte membrane arranged between the anode and the cathode through the medium of electrodes called separators (also called bipolar plates). The separators have grooves acting as channels for a gas (e.g., hydrogen or oxygen). The fuel cell may exhibit an increasing output by increasing the number of cells per stack.
The separators for fuel cells (fuel cell separators) also act as parts for recovering a generated current to outside of the fuel cell. Materials for separators have to maintain a low contact resistance over a long duration during use as separators. The contact resistance is a resistance which causes voltage drop due to an interfacial phenomenon between the electrode and the separator surface. The separators also should have satisfactory corrosion resistance, because the inside of fuel cells is an acidic atmosphere.
To meet these requirements, separators milled from molded articles of graphite powders, and separators molded from a mixture of graphite and a resin have been proposed. These separators, however, have inferior strengths and toughness and may be broken upon application of vibration or impact, although they have satisfactory corrosion resistance. To avoid these disadvantages, various types of separators prepared from metallic materials have been proposed.
Exemplary metallic materials having both corrosion resistance and conductivity include gold (Au) and platinum (Pt). Specifically, customarily-studied techniques employ a metallic substrate made from a metallic material capable of having a small thickness and exhibiting satisfactory workability and high strengths, such as an aluminum alloy, stainless steel, nickel alloy, or titanium alloy, in which the metallic substrate is coated with a noble metal such as Au or Pt to give separators having both corrosion resistance and conductivity. The noble metal materials are, however, very expensive and cause higher cost.
As a possible solution to these issues, metallic separators without using noble metal materials have been proposed.
Typically, exemplary proposed separators include a separator including a substrate and a carbon film formed on a surface of the substrate by vapor deposition (see Patent Literature (PTL) 1); and a separator including a stainless steel substrate and graphite compression-bonded on a surface of the substrate (see PTL 2 and PTL 3).
Exemplary proposed separators further include a separator including a metallic substrate and, formed on a surface thereof, a carbon layer having a G/D ratio of 0.5 or less as measured by Raman spectroscopy (see PTL 4); and a separator including a metallic substrate and, formed on a surface thereof, a carbon layer including an amorphous carbon layer and a graphite region (see PTL 5).